I thin it depends on the child.. why not trial it for day sleeps and see how he goes..

then on the odd night try it for the night.. if you have room for the cot and a bed that is..

my Ds2 went into a normal sized bed at 16 months and my Ds3 went into a cot bed at 21 months.. he coudl have gone earlier but we didn't have the bed lol..